Low IR value

[Muhammad A]

HI ALL.I have to powrr up refrigeration compressor 120hp motor.on Megger test I am getting 1MOhm IR values for compresor motor( connected in circuit(magnetic contactors, but deenergized) ..Can I power up with this value? What could be reason and possible solution?

 

[waross]

My procedure when faced with a low Megger reading:
277 Volts/1,000,000 Ohms = 0.000277 Amps
277 Volts x 0.000277 Amps = 0.0767 Watts
0.0767 Watts x 3 phases = 0.23 Watts loss due to insulation leakage.
I can live with that, but.
I recommend testing at the motor leads with the feeder disconnected if possible.
At least testing with the contactor disconnected. You may be seeing faint tracking at the contactor.
Trend testing, every week or every month or so.
Trend testing should be done:
a) When the motor is hot, immediately after running normally with normal load for several hours. (Best)
or
b) When the motor is cool after standing stationary overnight. (good)
Note: First determine if the low reading is from the motor or from the contactor or from the feeder.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

If I remember right the NETA standards have a minimum IR value of 1 Mohm per kV + 1. If you use that rule of the thumb your compressor motor is acceptable, but just barely.

 

[che12345]

I would like to propose the following for your consideration.
1. Disconnect the cables on the motor terminals. Measure the motor insulation resistance and polorisation index terminal on A-F, B-F and C-F . These test would give an indication the health of the motor alone, excluding the feeder cables.
1.1 Take the same measurements under the same temperature condition in say six month, to see the trending.
2. In addition to the cables disconnected from the motor terminals, disconnect the cables on the contactor. These test the health of the cables, excluding the motor.
a) measure the insulation resistance L1-E, L2-E, L3-E and L1-L2, L1-L3 L2-L3.
3. BTW a) for a LV motor, test with voltage 500V.
b) IR of 1 Mohm is on the LOW side. If it is running fine, go ahead but take/prepare for remedial measure ASAP.
Che Kuan Yau (Singapore)

 

[electricpete]

What could be reason and possible solution?

If the motor was just shut down, you could be testing at a winding temperature above 40C=104F. Any limit you find for motor insulationi resistance should include temperature correction. I'm going by memory but I think IEEE43 recommends 5megaohm minimum for readings temperature corrected to 40C.

As Bill mentions, you are not testing the motor alone unless you disconnect it.

Ambient conditions (humidity) can make a big difference.

Winding conditions (wet, contaminated) can make a difference.

Space heaters may not be working correctly. (check them).

Check polarization index for additional information. If low it generally points towards moisture.

If the winding configuration allows splitting the winding and testing 3 phases independently, then do that. If one phase is much different than the others that heightens the concern.

Was the motor running properly before the test (if yes that generally decreases the level of concern). If it is a new motor can you trace it's history back to last good test at factory, shop or warehouse and how long ago and where stored since then.

In the end it may end up being a judgement call. Do you go to additional trouble to investigate (potentially disassemble, clean , bake, retest) or do you put it in service and see what happens. In some case it depends on the situation and your role in everything (who is assuming the risk). If you're a contractor you probably don't want to accept the risk on behalf of the customer, tell them how it compares to the standard and whatever other info you have gathered and pass any decision to accept the risk or spend more money to investigate / remediate on to them.

 

[waross]

Hermetic? Semi hermetic? or external motor drive.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!